The last few decades have seen significant efforts expended in exploring possible therapeutic uses of guanine analogs and nucleosides thereof. A number of nucleoside analogs are currently being marketed as antiviral drugs, including HIV reverse transcriptase inhibitors such as AZT, ddI, ddC, d4T, 3TC and the guanosine nucleoside analog abacavir. While not adhering to a particular theory, nucleoside analogs may provide benefits by directly inhibiting the pathogen or tumor, by stimulation of host immune functions, or some combination of these or other mechanisms.
One of the studied guanosine analogs with demonstrated immunomodulatory activity is 5-amino-3-(β-D-ribofuranosylthiazolo[4,5-d]pyrimidine-2,7(3H, 6H)dione(7-thia-8-oxoguanosine). For example, certain pyrimido[4,5-d]pyrimidine nucleosides are disclosed in U.S. Pat. No. 5,041,542 to Robins et al. as being effective in treatment against L1210 in BDF1 mice. In addition, 3-β-D-ribofuranosylthiazolo[4,5-d]pyrimidines demonstrating significant immunoactivity, including murine spleen cell proliferation and in vivo activity against Semliki Forest virus, are disclosed U.S. Pat. Nos. 5,041,426 and 4,880,784 to Robins et al. A number of publications have also described non-glycosyl derivatives of the thiazolo[4,5-d]pyrimidine moiety. See, e.g., U.S. Pat. Nos. 5,994,321 and 5,446,045; Revankar et al., J. Het. Chem., 30, 1341-49 (1993); Lewis et al., J. Het. Chem., 32, 547-56 (1995).